1. Field of the Invention
This invention relates to a process for producing an unsaturated carboxylic acid from an alkane, in particular, a process for producing acrylic acid from propane by a single step vapor phase oxidation reaction with an excess of alkane relative to oxygen and with recycle of the unreacted alkane and the alkenes produced as byproducts of the reaction, particularly propylene.
2. Description of the Prior Art
The production of an unsaturated carboxylic acid, such as acrylic acid or methacrylic acid, is accomplished commercially by catalytically reacting an olefin, such as propylene or isobutylene, with oxygen to form an alkenylaldehyde, such as acrolein or methacrolein, which is subsequently catalytically reacted with oxygen. There are instances of producing acrylic acid and other unsaturated carboxylic acids from propane and other alkanes in a one step vapor phase catalytic oxidation reaction. Alkanes, such as propane, have advantages of cost and of availability over olefins.
Vapor phase catalytic oxidation of alkanes will, in addition to the unsaturated carboxylic acid, produce byproducts, including an alkene or olefin. For example, when the alkane is propane, byproducts of carbon monoxide, carbon dioxide, acetic acid and propylene will be formed. The olefin may be separated from the other byproducts and recycled into the reaction zone where the olefin can be converted into an unsaturated carboxylic acid, e.g., propylene into acrylic acid. In the alternative, the olefin may be separated from the other byproducts and converted to an unsaturated carboxylic acid in a separate process using known catalysts for converting an olefin into an unsaturated carboxylic acid or used in other processes to produce other products.
U.S. Pat. No. 6,642,174 discloses a mixed metal oxide catalyst for conversion by vapor phase oxidation of an alkane or a mixture of an alkane and an alkene to an unsaturated carboxylic acid or, in the presence of ammonia, to an unsaturated nitrile. The process for oxidation of propane or isobutane to acrylic acid or methacrylic acid may be practiced in a single pass mode or a recycle mode where at least a portion of the reactor effluent is returned to the reactor. The molar ratio of alkane or alkane/alkene mixture to oxygen in the starting material gas is disclosed as 1:0.1 to 10, more preferably 1:1 to 5.0.
U.S. Pat. No. 6,492,548 discloses processes for the oxidation of an alkane, such as propane, to form unsaturated carboxylic aldehydes and acids, such as acrolein and acrylic acid, in three steps, the first converting the alkane to the corresponding alkene and then converting the alkene to the corresponding unsaturated aldehyde and then converting the aldehyde to the corresponding unsaturated carboxylic acid. Unreacted propane, propylene, oxygen, carbon monoxide and carbon dioxide may be recycled to the propane-to-propylene reaction. Operating at low propane-to-propylene conversion makes the selectivity to propylene unexpectedly high and the presence of propane was found to enhance the efficiency of the propylene-to-acrolein reaction. The low-conversion, high-selectivity mode of operation is highly efficient provided that unreacted propane is recycled to the propane oxidation reactor. Propane and other noncondensed gases may be recycled without significant additional purification steps. The ratio of propane to oxygen is disclosed as being in the range from 5:1 to 40:1.
U.S. Pat. No. 6,441,227 discloses a two stage process for the production of alpha-beta carboxylic acids through catalytic vapor-phase oxidation of olefins with mixed metal oxide catalysts by oxidizing an olefin, such as propylene, to an aldehyde and then oxidizing the aldehyde to a carboxylic acid. Propane may be present as inert gas.
U.S. Pat. No. 6,114,278 discloses a mixed metal oxide catalyst containing Mo, V, Ga, Pd and Nb for a one-stage catalytic vapor phase partial oxidation of propane for production of acrylic acid. Non-reacted initial reactants may be recycled but less than 1% propylene is intended to be formed, more preferably, no detectable propylene is formed. The ratio of propane to oxygen is disclosed in the range of 1/5-5/1. Molar ratios for propane:oxygen:nitrogen were 20:10:70 in the working examples.
U.S. Pat. No. 5,380,933 discloses a method for producing an unsaturated carboxylic acid from an alkane by vapor phase catalytic oxidation with a mixed metal oxide catalyst. Recycle of propylene is not disclosed. The alkane:oxygen:diluting gas:H2O in the starting material gas is disclosed as preferably being 1:0.1 to 10.0:0 to 20:0.2 to 70, more preferably 1:1 to 5.0:0 to 10:5 to 40.
U.S. Pat. No. 6,291,393 discloses a metal oxide catalyst for producing acrylic acid by vapor phase catalytic oxidation of propane, propylene or acrolein. Unreacted propane and propylene as an intermediate may be recycled. The volumetric proportion of air to propane is disclosed as preferably 30 times or less, more preferably from 0.2 to 20 times.
U.S. Pat. No. 6,162,760 discloses a process for the ammoxidation of paraffins to unsaturated mononitriles in which unreacted propane and isobutane along with propylene and isobutene produced by the reaction are recycled for conversion to acrylonitrile and methacrylonitrile. The feed composition has a mole ratio of the paraffin to ammonia in the range of from about 2.5 to 16 and a mole ratio of paraffin to oxygen in the range of from about 1.0 to 10. The catalyst is a vanadium-antimony oxide catalyst containing at least one of titanium, tin, iron, chromium or gallium and at least one of molybdenum, tungsten, niobium, arsenic, tellurium or selenium.
U.S. Pat. No. 6,043,185 discloses a process for the ammoxidation of paraffins to unsaturated mononitriles in which unreacted propane and isobutane along with propylene and isobutene produced by the reaction are recycled for conversion to acrylonitrile and methacrylonitrile. The feed composition has a mole ratio of the paraffin to ammonia in the range of from about 1.0 to 10, preferably 2 to 4, and a mole ratio of paraffin to oxygen in the range of from about 1.0 to 10, preferably 1 to 3. The catalyst is a molybdenum-antimony-gallium mixed metal oxide catalyst.
U.S. Patent Application Publication no. 2002/0123647 discloses a method for producing an unsaturated carboxylic acid from an alkane with a mixed metal oxide catalyst. The unreacted alkane and the generated alkene are recycled to the reaction zone. An analogous method for the preparation of unsaturated nitriles is also disclosed. The molar ratio of alkane or mixture of alkane and alkene to oxygen to diluting gas to water is disclosed as (1):(0.1 to 10):(0 to 20):(0.1 to 70), preferably (1):(1 to 5.0):(0 to 10):(0.2 to 40).